Electrostatic loud speaker



June 11, 1935. A. A. THOMAS ELECTROSTATIC LOUD SPEAKER Filed April 5, -1929 R O T. N E V m Patented June 11, 1935 UNITED STATES PATENT OFFICE ELECTROSTATIC LOUD SPEAKER Application April 5, 1929, serialV No. 352,642'

2 Claims..

My invention is for an electrostatic loudspeaker provided with a pair of diaphragms adapted to operate most efciently on different bands of frequencies, so that the entire audible scale of tones is faithfully reproduced. A large diaphragm accentuates the lower frequencies, and a small diaphragm emphasizes the higher frequencies. For this purpose, each condenser unit is subjected to potential variations in a circuit adapted to pass certain bands of frequencies, The two diaphragms are preferably mounted in tensioned condition 0n .Opposite sides of a flat frame in electrostatic relation to a pair of stationary plates. The supporting `frame may be rotatable on a base, so that the diaphragms can be turned in the desired direction.

The novel features and practical advantages of my invention will be understood from a description of the accompanying drawing, in which- Fig. 1 shows a face view Yof a condenser speaker constructed .in accordance with my invention, certain parts being broken away for clearness and lack of space.

Fig. 2 represents a vertical section on line 2-2 of Fig. l;

3 is an enlarged fragmentary section to show certain structural details;

Fig. 4 is an enlarged bottom view of the central portion of the supporting base or stand; and

Fig. 5 is a diagram of circuit connections.

A circular frame III is provided on opposite faces with a pair of circular recesses I2 and I3. in which are mounted fixed condenser plates or disks I4 and I5. The frame I0 may be molded as a single piece of insulating material, such as bakelite, earthenware, fiber, hard rubberand others along the same line. The condenser plates I4 and I5 may conveniently be cut or stamped from sti sheets of metal like aluminum, duralumin, brass and other metals and alloys that are familiar to the electrical expert. If the condenser plates I4 and I5 are inserted in their respective recesses with a tight frictional t, ne additional fastening means are necessary. It will be noticed in Figs. 2 and 3 that the surrounding wall of each recess projects slightly beyond the outer surface of the associated plate. The purpose of this feature will later be explained. In the present embodiment, the condenser plate I4 is considerably larger than plate I5, so that diaphragms of correspondingly different size may be mounted in operative relation to the fixed plates to improve the reproduction (Cl. 18h-31) ofthe instrument. This will be understood as the description proceeds.

The circular recesses I2 and I3 in frame lIll are surrounded by a pair of annular grooves I6 and Il, which are preferably tapered or conical in cross-section. Two vibratory electrodes I8 and I9 are mounted on the opposite faces of frame III over the fixed plates I4 and I5. A simple way to fasten the vibratory electrodes I8 and I9 in place is by means of rings 20 and 2|, 10 which engage the edges of the electrodes and are secured to the frame by screws 22 or otherwise. The members I8 and IS are thin elastic membranes of metal adapted to operate as loudspeaking diaphragms. Among materials suitable for this purpose I may mention nickel, invar (an alloy of nickel and steel), duralumin, bronze, steel, and similar metals and alloys. A dia` phragm of invar has the advantage that it is not affected by changes in temperature. In the present instance, the inner side of each membrane is covered with insulation 23, which may be silk, cheesecloth, netting, rubber cement, a sheet of India rubber, and other materials along the same line. Instead ofv providing the vibratory membranes I8 and IS with an insulating sheet or coating, the adjacent faces of the fixed condenser plates I4 and I5 may be insulated in the same mannery to prevent short-circuiting of the instrument during the vibration ofthe membranes.

The membranes I8 and I9 are mounted yin stretched condition by the peripheral rings `20 and 2|, but it is desirable to increase the tension of the membranes to a greater degree than can ordinarily lbe obtained by the clamping rings alone. For this purpose I use a pair of wedgeshaped rings 24 and 25 adapted to fit into the.V correspondingly shaped grooves I6 and I1. Screws 26 or other fastening means hold the rings 24 and 25 firmly in place. It is clear from Figs. 2 and 3 that, when the wedge-shaped rings 24 and 25 are forced into the grooves I6 and I1 over the membranes I8 and I9, the latter are drawn into the grooves and thereby stretch radially in all directions to the desired 'degree ofk tension. This may be regulated by the position of the rings, the tension of the membranes being greatest when the rings are forced fully into the grooves. I have previously mentioned that the shoulders around the circular recesses I2 and I3 project slightly beyond the condenser plates I4 and I5. The purpose of this construction is to maintain the tensioned membranes, I8 and I9 spaced a predetermined amount from the adjacent fixed plates, as indicated at 21 in Fig. 3. It will be understood, of course, that this iigure is greatly exaggerated, particularly as to the thickness of the Vibratory membrane and its separation from the fixed electrode.A The air space 21 is not absolutely necessary, because the two condenser elements of each unit needV only Ibe separated by the insulating sheet or covering 23, which in that case should be suiciently elastic to permit free vibration' of the membranes under the action of electrostatic attraction and repulsion. The metallic membranes I8 and I9 may also be formed by coating the tensioned insulating sheets 23 on the outside with metal foil or paint, such as are available in the market. v y

The circular supporting frame III is provided with a pair of peripheral flanges 28 for receiving screws 29, which hold a pair of screens 30 in place. cloth or suitable grilles surrounded each by a ring 3I through which the screws 29 pass. .The purpose of screens 39 is not only. to hide the membranes I9 and I9, but to prevent a person from accidentally. touching the charged electrodes. It is understood, of course, that the screens or grilles 39 are suiiiciently open to permit the free passage of sound. In some cases it may be desirable to mount the circular frame I rotatably on a suitable base or stand 32. In Fig. 2 there is atubular mounting 33 iixed at its upper end in frame IIJ and at its lower end projecting rotatably into a hub or bushing 34 in base 32. The tube 33 has a lateral i'lan'ge or shoulderV 35, which rests on the bushing 34 to permit easy rotation of frame I9 and ltube 33.` A crosspin 36 in the lower end of tube 33 engages a stop 31 in base 32 toY limit the rotary movement of frame IU. The advantage of the rotary mounting of frame I0 is that the diaphragms I8 and I9 can' be turned in any desireddirection. The various conductors which are electrically connected to the condenser elements are concealed in a passage I 9 in frame I0 and pass through the tube 33 intothe base.

A simplified diagram of circuit connections for the condenser units I 4|8 vandl I5-I9 is illustratedin Fig. 5. A generator 38 of substantially constant potential is connected at on'e side to the iixed condenser plates I4 and I5, and at the other side to the Vibratory membranes I3V and I9. Inductance or resistance devices 39 of the required electrical values are inserted in the polarizing circuit to prevent the Variable signal voltage from' passing through the generator circuit. It will be understood that 38 represents any suitable source of polarizing voltage for the condenser elements, such as a storage battery or the houselighting circuit. If thev latter operates on alternating current, it will, of course, be necessary to interpose a rectiiier. The signal voltage impressed on the condenser elements is supplied in this instance through a transformer 40 having a` primary coil 4I 'and two secondary coils 42 and 43. f The primary coil 4I isconnected in the output circuit of a suitable ampli- These screens may be loosely woven fying system, many examples of which arey known in the radio art. The opposite sides of coil 42 are connected lby wires 44 and 45 to the condenser elements I4 and I8, respectively. One side of secondary coil 43 is connected by wire 46 to the fixed plate I5, and the other end of the coil is connected by wire 41 to the vibratory membrane I9. A condenser 48 is shunted across the secondary coil 42,'and an inductance 49 is connected across Vthe secondary coil 43. The electrical values of devices 48 and 49 are sovchosen that low frequencies of potential Variation arey impressed on the condenser units tively, the lower and upper portions of they musical scale. The diametersV and tension of the two diaphragms IS'and I9 are so chosenV that the rstdiaphragm has a low natural frequency and the second diaphragm a high naturalV frequency. It is not necessarythat the diaphragms I8 and I9 shall always'be of different sizes, because they may have the same ,di-v

ameter and yetV be capable of accentuating dif*- ierent bands of frequencies. Asacoustical experts know, therquality' of` sound produced by a diaphragm 4depends tosome extent on the material and elasticity of the diaphragm. yThe membranes I8 and I9 need not be circular in all cases, for they can be made rectangular, ink

lwhich 4event ythey would( be supported runder tension at twoopposite ends. 'Although l. have shown and described a spe- Vciiic construction, I want it understood that my invention is not limited to the detailsk set forth.

It is to be expected that changes and modifications will occur to others ,without departing from the scope of the invention as deiined in the appended claims.

I claim as my invention:

1. An acoustic device, comprising in combina-V tion, a` housing including a disc shaped member, a diaphragm, supporting members attached to said housing for supporting said diaphragm, and a grille mounted opposite said housing for protecting the diaphragm, supports for said grille, said supports for the grille being inde- ADOLPH A. THOMAS. 

